1. Field of the Invention
The present invention relates to integrated circuits comprising components formed in active semiconductor areas and, more specifically, is integrated circuits in which reflective layers are placed under the active semiconductor areas.
2. Discussion of the Related Art
Conventionally, integrated circuits are divided into several active areas formed in a semiconductor substrate. In the case of integrated circuits intended to detect images, each active area or pixel comprises at least one photon-capture layer, typically a lightly-doped photodiode layer, and an associated transfer transistor. When the circuit is illuminated, the photons of the incident light beam penetrate into the capture layer and create electron/hole pairs therein. The electrons are transferred to an electronic circuit by the associated transfer transistor.
The depth of the capture layer at which most of the electron/hole pairs are formed depends on the wavelength of the incident light beams. For example, if the circuit is illuminated by a blue light beam (short wavelength), most of the electron/hole pairs form in the first micrometer of the capture layer. Conversely, in the case of a red light beam, most of the electron/hole pairs form in the first three or four micrometers of the capture layer.
To improve the image detection, it has been provided to place a reflective layer under the different active areas. When a photon originating from a light ray of large wavelength penetrates deeply into the capture layer, it is sent back to the surface by the reflective layer. The associated electron/hole pair is then formed as the photon returns up to the photodiode. This enables increasing the probability for the electron to be captured by the photodiode.
U.S. Pat. No. 2006/0118897 A1 describes a method for forming a structure comprising a photodiode under which is placed a mirror. In this method, it is started from a structure of silicon-on-insulator type (SOI). Walls for separating and insulating the different active areas are formed in the silicon layer and in the insulating layer. The insulating material is then etched, via openings formed in the silicon, and a reflective material is deposited in the etched space. The different elements of the active areas (photodiodes and transistors) are then formed in and on the silicon layer located above the reflective material.
The above-described method has the disadvantage of requiring an SOI-type structure. Further, this method does not allow the forming of reflective layers under different silicon thicknesses.